It is known that the traditional system for laying an electric line provides a so-called “braked” laying, using machines with multiple capstans: a winch machine, which recovers a steel cable with a drawing function, and a braking machine, which applies a controlled tension on the wires during the laying step, so as to guarantee they can remain suspended to the guide devices provided on the intermediate supports, so as not to slide on the ground or against obstacles present under the wires, such as road or rail crossings, intersections with other electric lines and so on. Moreover, it is more and more common to use a helicopter to draw the pilot cable from the brake station to the winch station, or sometimes even to draw the final wire, thus eliminating the winch machine in this operation.
The same concepts are applied to aerial laying of guard cables with optical fiber cores (OPGW) or without optical fiber cores (GW).
The technology of braked laying is also applied in laying the electric overhead contact lines for railway traction, with the difference that the braking machine is mounted on a translating wagon, whether self-propelling or drawn, and thanks to the movement of the wagon the brake releases the contact cable or the suspension cable at a controlled tension.
The wires unwound and braked by the braking machines come from reels, where they are wound during the production step. During the laying step the reels are supported by cable winding and unwinding units, called reel-bearing trestles. The trestles are normally equipped with positive action disc brakes, so can be adjusted from the outside, and/or with hydraulic drive of the braking system, normally by means of a kinematic chain consisting of a hydraulic motor, brake and gear transmission, or mechanical reduction gear. This is because the wire branch comprised between reel and capstans of the braking machine must be kept braked at a reference value, normally comprised between about 150 and 250 kg, so as to guarantee the so-called “counter-traction” needed to generate friction of the wire on the pair of capstans, and hence to keep the wire under tension, at the value of the laying traction, once it has come out from the capstans of the braking machine.
Traditional braked laying machines are known to be equipped with an automatic drive mechanical safety device called negative brake, the purpose of which is to hold the load of the wire or cable laid, both when the machine is stationary, that is, when it is not fed, and also in emergency situations, for example due to a malfunction in the circuit.
The negative brake is intended to hold a load in static conditions, not to function as a traditional brake, that is, with a progressive braking maintained under dynamic conditions. If the applied braking limit is exceeded, this causes a relative rotation of the fixed and mobile elements of said device, which after a certain time creates overheating and accelerated wear of the friction elements, precisely because the negative brake is not intended to work under dynamic conditions.
Traditional winch machines for braked laying are also normally equipped with two devices to limit and control the overload, which if used correctly can prevent situations of serious overloading along the path of the cables laid.
Using a mean other than traditional winch laying machines to perform the traction and/or translation operations, as in the case of the railway wagon or helicopter, means that a possible sudden stoppage of the braking machine, even if due to extraordinary causes, causes the negative brake to close, which is the automatic drive safety device to hold the load. This closure of the negative brake generates a recoil on the translating mean, not necessarily synchronized with the braking machine or having a long stoppage time with respect to the instantaneous closure of the negative brake, and a consequent overload on the stretched cable, which can lead to damage and even breakage of the cable, with obvious risks to the safety of the operators and risks of great damage to everything that is under the cable. Therefore, in practice, this recoil neutralizes the safety function for which the negative brake is supplied. If a helicopter is used, the recoil of the cable could even lead to the instability of the aircraft, with obvious serious consequences.
The same can be said in the event of a sudden stoppage of the winding/unwinding machine connected to the braking machine, on which the cable to be laid is positioned and which must supply the correct tension (called counter-traction) to the wire, which is needed to generate the friction of the wire on the pair of capstans.
If the braking machine stops, a normal traction regulation valve present in known systems, with a hydraulic or electric piloted command, moves to a position of maximum opening, that is, minimum pressure of the regulation field, once the piloting stops, so that traditional systems with an accumulator to keep the negative brake open lose their effectiveness, because the tension on the cable or wire is no longer controlled.
From the Italian patent n. 0001414902 a safety plant is known for a cable-laying machine which is provided with a motorized drive device connected to the valve to regulate the traction or maximum pressure of the plant, and having the function of keeping the traction set where other devices prevent the automatic closure of the negative brake, hence limiting the recoil and keeping the main valve for regulating the traction under pressure, with the final result of keeping the wire under tension and preventing it from falling to the ground. The plant described in this patent therefore acts on controlling the traction applied to the machine.
However, this plant does not behave in the best way nor is it perfectly effective in particular situations of the machine, such as for example malfunctions or other anomalies that can occur in the work cycle of the machine, such as malfunctions of the electric, electronic, hydraulic type, or motor breakdowns. Moreover, the plant is not optimal from the point of view of safety, and does not intervene on external devices like the support trestle for the reels of the laying machine.
The unit for winding and unwinding cables, or reel-carrier trestle, can also be provided with its own negative brake, which is generally a negative lamellar brake in an oil bath. The brake acts under the thrust of a series of springs on pairs of alternate discs, fixed and mobile. The brake is released due to the effect of the hydraulic pressure in the piston of the negative brake. Negative lamellar brakes in an oil bath are used for so-called static braking or as normal parking brakes. In substance, using said brakes, the unit for winding and unwinding cables, in the event of malfunctions, is stopped suddenly, entailing both safety risks with regard to operators, and also risks of damage to the equipment of the laying apparatus.
One purpose of the present invention is therefore to obtain a unit for winding and unwinding cables for cable-laying machines which is provided with a braking device that does not stop the machine suddenly and is able to guarantee a minimum braking in a dynamic condition, which therefore allows to continue and/or complete the laying operations according to a suitable logic and intervention procedure, before the unit for winding and unwinding cables is safely stopped.
Another purpose of the present invention is to obtain a unit for winding and unwinding cables for cable-laying machines which is provided with a dynamic braking device that does not overheat and which is long-lasting.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.